Testing circuit



G. KOEHLER Jan. 5, 1943.

TESTING CIRCUIT Filed Dec. 8, 1941 Q/Zye/z for' if um @l a Paiented Jan.. 5, 1943 TESTING CIRCUIT Glenn Koehler, Madison, Wis., assignor to Chicago Transformer Corporation, a corporation of Illinois Application December 8, 1941, Serial No. 422,067

2 Claims.

This invention relates to a testing circuit and particularly to a testing circuit for use with a device or means having a voltage input and output whose ratio over a band of frequencies is to be determined. In particular, the circuit is useful in connection with the determination of characteristics of transformers, ampliiler stages, illter systems and the like. The circuit may advantageously be used in testing audio frequency transformers and amplifiers. When used in this manner, the significant test results may be logarithmlcally related to yield relative decibel values. It is thus possible to plot a curve showing decibel to frequency relationship. Other fields of use will occur to those skilled in the art.

Referring now to the drawing, the single iigy ure shows the circuit.

Terminals I and may be considered as the source of alternating currents at variable or predetermined frequencies. Thus an oscillator may supply terminals I0 and with such alternating currents. The frequencies may have any values and ranges and as applied to testing audio frequency transformers may vary from about 30 cycles per second up to 10,000 or even higher.

In order to simulate true operating conditions for the device being tested, an artificial driver impedance i2 may be connected to terminal I0. Impedance i2 is then connected to the device tested, here primary i3 of an audio frequency transformer It, and the input circuit is completed by wire I5 connecting primary |3 and terminal l. In the case shown, impedance I2 may be the output circuit of some amplier or other tube.

Transformer i4 has the secondary i6 supply output terminals Il and i8. Output terminals Il and i8 may have a terminating impedance 20 connected or not, as desired. At any rate terminals Il and i8 are connected together. Terminal Ill is connected by wire 2| through a coupling condenser 2|' to one end 22 of a resistance 23 whose terminal 24 is connected to a grounded wire 25 connected to terminal Il. Cooperating with resistance 23 and forming a potentiometer is a slider 26 connected to point 21 of a three position switch 28. Rotor or movable contact 29 of switch 281s connected by wire 30 to control grid 3| of a vacuum tube ampliiler 32. This ampliiler may be of any suitable type and as disclosed herein is of vthe pentode variety for obtaining high voltage amplication. Thus tube 32 has its cathode 33 connected through a bias resistor 35 to wire 25. A bias control may be obtained by any desired portion of resistance 35. An accelerating grid 40 is connected by wire 4| to a resistor 42 whose terminal 43 is to be connected to a positive source of potential. Resistor 42 5 also functions to leliminate any tendency for oscillations through feed back in tube 32 (and later described tube |32) as well as between these two tubes. A suppressor grid 45 is connected back to the cathode while an anode 46 is connected by wire 4l to a load resistor 48 and thence to a terminal 43. It is understood that separate voltage connections for the various electrode connections may be provided if separate potential sources are to be used. However, the connectlons shown are common and convenient.

Anode t6 is also connected by wire 41 to one terminal oi a blocking condenser 50, the other terminal oi which is connected to a switch point 5| of a switch 52. 'Movable contact 53 cooperating with point 5| is itself connected by wire 34 to a bias resistor 55 having a terminal 58. Resistor 55 is also connected to control grid 51 of a vacuum tube detector 5B. This tube has a cathode 59 connected by wire 60 to a terminal 5I. Between terminal 6| and terminal 56, a variable bias means as a batteryis disposed with terminal 8| positive to terminal 56. It is understood that a voltage drop due to the anode supply' may be used for `grid bias as is customary in modern radio receivers. Anode 63 is connected by wire 64 through a meter 65 and resistor 68 to terminal 61, adapted to be connected to the positive pole of a plate supply.

Going back to terminal l1, lead |2| is connected S6 through coupling condenser |2|' similar to condenser 2I' to terminal |22 of a resistor |23 whose terminal |26 is connected to wire 25. Potentiometer contact |26 is connected to switch point |21 of a three position switch t28 similar to switch 28. Switch points 80 and |80 on the two switches are connected to Wire 25. Switch points 8| and |8| on the two switches are connected together by wire |82 and are connected to switch point 21 by wire 83. Rotor |29 of switch |28 is preferably coupled to move with rotor 29 of switch 28 so that the switch contacts are established as indicated.

Rotor contact |29 is connected to control grid IBI of a vacuum tube amplifier |32, similar in characteristics to tube 32. Cathode |33, suppressor |45 and anode |46 are connected through the variously numbered elements in a manner analogous to the corresponding parts in tube 32. Accelerating grid |40 is connected by wire |4| slider 36 connected to wire 25 and cutting out 55 to wire 4| and these two are connected to a by- 255 is connected to grid |51 of detector |58 hav- Y ing characteristics similar to detector 58. Cathode |59 is connected by wir |68 to terminal 6| while anode |63 is connected by wire |64 to a variable resistor |65 and thence through resistor |66 to terminal 61.

Sliders 85 and |85 cooperate with resistors 66 and |66 respectively and are connected by wires 88 and |86 to resistance 81 and meter 88 both in series with each other. Switch points 98 and |96 on the two position switches are grounded to wire 25 by wires 8| and |9|.

With switches 52 and |52 in the position shown, the circuit may be balanced to compensate for lack of symmetry du'e to differences between tubes 5'8 and |58. In this position, control grids 51 and |51 are grounded to wire 25. Hence reslstance |65 and sliders 85 and |85 may be adjusted to balance tubes 58 and |58 so that meter 88 does not show current flowing in either direction. Resistance |65 functions more or less as a balancing impedance for meter 65 as well as to balance the characteristics of tubes 58 and |58. Under some conditions, resistance |65 may even be disposed in the anode circuit of tube 58. In practice resistance |65 and slider 85 are the main controls for obtaining a balanced condition in meter 88. It is clear that meter 85 is merely an ordinary milliamrneter while meter 88 should read both positive and negative current or poteny tial values.

Assuming that meter 88 shows no current and meter 65 shows a proper space current for the tubes, switches 52 and |52 may now be moved to engage contacts 5| and |5|. Control grids 3l and |3| are connected to contacts 8| and |8| and control grids 51 and |51 are connected to their respective blocking condensers. If meter 88 shows a reading, then the grid biases of tubes 32 and |32 may be adjusted by operating with slides 36 and |36 and resistance |65 and slider |85 are adjusted so that meter 88 reads no current or voltage over the complete range of input voltages to tubes 32 and |32. After this is done it may be advisable to go back to the ilrst switch position: i. e. contacts 88 and |80 connected to rotors 58 and |53. This is merely to recheck the balance of tubes 58 and |58 and may be required merely because of coupling through common power supplies.y etc., between the successive amplifier stages.

Assuming that the tubes have been properly balanced, switches 52 and |52 are retained in their working positions to connect contacts 5| and 53 as one pair and |5| and |58 as another pair. Switches 28 and |28 are now moved so that contacts 8| and |8| are connected to their respective rotors. At the same time terminals I8 and have impressed thereon currents at predetermined frequency and potential. Hence input (with reference to transformer I4) voltages are impressed across resistance 23. A predetermined voltage ratio is taken by slider 26 and the same voltage impressed upon both control girds 3| and |3|. It is clear that meter 88 will remain balanced in zero-position while meter 65 will show a definite reading.

-By the above procedure an articlal or arbitrary output ievel is taken as anorm and deviations therefrom may now be measured. To do this, switches 28 and |28 are moved to the testing position to engage points 21 and |21. The same input frequency and voltage at transformer I8 is still used. Slider |26 may now be adjusted so that meter 88 is balanced and shows no current i'low. Since resistors 23 and |23 may have different values, and since the transformer secondary I6 will feed resistor |23, it follows that slider |26 must be independently adjusted from slider 26.

Assuming now that slider |26 is properly positioned the testing of transformer I4 may now begin. Input frequency may now be varied. Thevoltage input may be maintained at a steady value by reference to meter 65. If the input at I0 and I is held constant over a frequency range, meter 65 should have a constant reading. The secondary response will have a substantially constant'proportion thereof impressed on control grid 3|. A substantially constant proportion of impressed primary voltage will be impressed on control grid 3|. If the transformer secondary output is constant over a frequency range, meter 88 will not move.

The object of dividing a portion of resistance |55 into decibel steps is to be able to have the characteristics of the transformer to change from dropping to rising without the necessity for reversing meter 88. When the characteristics rise above normal, slider 256 is lowered a deiinite decibel amount to bring meter 88 on an up scale position. Then the amount of change on 255 in decibel is subtracted from the decibel reading of the meter to get the decibel rise in characteristics of the transformer. This eliminates the necessity of reversing meter 88 or reversing the input terminals when the characteristics change from dropping or rising or vice versa.

Also by having the scale of meter 88 marked in logarithmic ratio and assuming the meter itself is of the usual type having a response directly proportional to the voltage or current, it

is clear that the characteristic ratio obtained are in'logarithmic ratio. Thus the decibel characteristics of a transformer may be determined in this manner also.

It is also possible to move slider |26 and control the energy in the portion of the system below line 25 so that meter 88 is balanced at all times. Hence the resistance ratlos of the settings of slider |26 may be utilized to determine the nature of the transformer output. Naturally if less resistance in |23 is necessary between |22 and |26 it follows that the transformer output is less in comparison to a previous setting. By proper markings on the scales for sliders 26 and |26, the characteristic curve may be plotted. If slider 26 is not moved during the entire test, then the relative values of positions of slider |26 only are significant. On the other hand, it is possible that some unusually high peak or low dip in transformer output may require a relocation of slider 26 in order to keep slider 26 on scale. In that case, a proper proportional factor will result from the ratio of new to old settings of slider 26. However, by making resistance |23 quite large, a long range of variation may be provided. Hence in the initial adjustments, slider |26 may be positioned near the middle of resistance |23 so that substantial variation around that position may be tolerated without change of position of 26. In actual practice, resistances 23 and |23 with their sliders may be any number of decade resistance boxes suitably calibrated either in ohms, decibels or arbitrary numbers in logarithmic or other proportion.

It is also possible to utilize both relative variations of sliders 26 and |26 and readings of meter 88. In such case, the resistance ratio must be multiplied by the meter reading ratio. However, it is simpler to utilize variations in one part of the system and keep the other part constant.

In order to have meter 88 move in one direction only,it is possible to operate one part of the system (with reference to wire 25) at a higher or lower energy level than the other. Thus for example, the output of the part of the system above line 25; i. e. (tubes 32 and 58) may be operated at a level, with reference to the rest of the tubes, so that meter 88 always has a positive reading, for example. In that case, the output of tube |58 will merely tend to decrease or increase the reading about the positive reading. In short, it merely makes a positive reading of meter 88 an artificial zero for meter 88 about which the meter can operate. In that case, an ordinary voltmeter may be used.

It is understood that the various resistors are non-inductive and that care be exercised in avoiding distortion in the amplifiers. The mere fact that tubes 32 and |32 are alike and balanced for example is not enough. Care must be taken so that in the range of operation of the tubes what might be termed the theory of functions should be preserved. Thus if a mere arithmetical difference between opposing tube voltages is to be measured, then the tubes should be operated only over the straight part of its characteristic curve. On the other hand, by operating on curved characteristics of tubes, nonarithmetical differences may be relied upon. Thus if a tube characteristic may be expressed by a simple exponential function it is clear that a pair of opposed tubes operating with such characteristics would immediately give voltage differences at meter 88 having logarithmic relations.

It is thus clear that in its simplest form the invention provides one or more opposed amplifiers through which an input and output may be fed. Resistance coupling is a simple means for obtaining satisfactory operation over a range of frequencies as well as insuring that each amplifier stage operates within prescribed conditions of voltage for fidelity. Apart from pick up resistors 23 and |23, it follows therefore that everything is symmetrical with respect to ground wire 25. In other words, resistors 48 and |48 are equal, resistors 55 and |55 are also equal as are condensers 50 and |50. Preferably the plate circuits of tubes 58 and |58 are symmetrical with respect to terminal 61.

In one respect, the invention provides a pair of opposed amplifiers whose anode circuits are connected in apparent push-pull relation. However the amplifier inputs are more or less independent with the magnitude of applied potentials functionally related to each other in accordance with the characteristics of a transformer or other device under test. The input voltages to both amplifiers are also in phase in the sense that both attain their peak values simultaneously rather than apart as in a true push-pull amplifier. The number of stages of amplification for handling each applied voltage may be varied to suit requirements. Obviously, the frequency range may also vary. Thus radio or intermediate frequency devices may also be tested by generally similar methods.

What is claimed is:

1. A system for comparing the relative voltages over a frequency band of the input and output of a device like a transformer, amplifier or the like, said system comprising a pair of vacuum tube amplifiers with each amplifier having a cathode, control grid and anode circuits respec tively, means for applying independently predetermined portions of the input and output voltages to the control grids respectively, means in the amplifier handling the input of the tested device for indicating the energy level of the system, means for combining predetermined portions of the amplified voltages in opposed relation to each other, means for indicating the differential in said opposed voltages, and switching means for cutting out the output voltage to the control grid of said amplifier and substituting therefore the same voltage as applied to the amplifier from the input whereby initial stabilization of the system may be obtained, and variations of input voltage over a frequency band may be compensated for.

2. In a system of the character described, a pair of vacuum tube amplifiers having cathode grid and anode circuits respectively, a meter in the anode circuit of one amplier, a balancing resistor in the anode circuit of the other amplifier, means for connecting the anode circuits in push-pull relation, means for applying potentials t0 the control grid having a definite relation to each other determined by attenuation characteristics of a converting device, and a potentiometer bridged between two points in the anode circuits for taking olf predetermined portions of the amplified potentials and combining them in opposed relation.

GLENN KOEHLER. 

